ter those asperities which project from the surface of the other ; and thus, un- 

 til these projections should be broken off, both wheel and axle must be moved 

 at the same time. It is on this account that, if the surfaces of the wheels and 

 axles are by any means rendered rough, and pressed together with sufficient 

 force, the motion of either will turn the other, provided the load or resistance 

 be not greater than the force necessary to break off these small projections 

 which produce the friction. 



In cases where great power is not required, motion is communicated in this 

 way through a train of wheelwork, by rendering the surface of the wheel and 

 axle rough, either by facing them with buff leather, or with wood cut across 

 the grain. This method is sometimes used in spinning machinery, where one 

 large buffed wheel, placed in a horizontal position, revolves in contact with 

 several small buffed rollers, each roller communicating motion to a spindle. 

 The position of the wheel W, and the rollers R R, &c, are represented in 

 fig. 24. Each roller can be thrown out of contact with the wheel, and restored 

 to it at pleasure. 



The communication of motion between wheels and axles by friction has the 

 advantage of great smoothness and evenness, and of proceeding with little 

 noise ; but this method can only be used in cases where the resistance is not 

 very considerable, and therefore is seldom adopted in works on a large scale. 

 Dr. Gregory mentions an instance of a sawmill at Southampton, England, where | 



the wheels act upon each other by the contact of the end grain of wood. The 

 machinery makes very little noise, and wears very well, having been used not 

 less than twenty years. 



The most usual method of transmitting motion through a train of wheelwork 

 is by the formation of teeth upon their circumferences, so that these indentures 

 of each wheel fall between the corresponding ones of that in which it works, 

 and insure the action so long as the strain is not so great as to fracture the 

 tooth. 



In the formation of teeth, very minute attention must be given to their figure, 

 in order that the motion may be communicated from wheel to wheel with 

 smoothness and uniformity. This can only be accomplished by shaping the 

 teeth according to curves of a peculiar kind, which mathematicians have in- 

 vented, and assigned rules for drawing. The ill consequences of neglecting 

 this will be very apparent, by considering the nature of the action which would 

 be produced if the teeth were formed of square projecting pins, as in fig. 25. 

 When the tooth A comes into contact with B, it acts obliquely upon it, and, as 

 it moves, the corner of B slides upon the plane surface of A in such a manner 

 as to produce much friction, and to grind away the side of A and the end of B. 

 As they approach the position C D, they sustain a jolt the moment their sur- 



